Astronauts on long missions to the Moon, Mars and the International Space Station may be at increased risk of developing infectious diseases, according to research showing that germs can become much more virulent than usual in space.

A study conducted on the space shuttle Atlantis has revealed that salmonella bacteria become about three times more likely to cause disease when allowed to develop in microgravity for just 24 hours.

As well as illuminating the infectious hazards faced by space crews, as Nasa prepares for manned trips to the Moon and Mars, the findings could have important implications for disease control on Earth.

The study has highlighted a genetic “master switch” for salmonella virulence that appears to cause the microgravity effect, which scientists say is a promising target for new drugs and vaccines.

It is well known from more than four decades of manned spaceflight that low gravity has an adverse effect on bone density, muscle tone and the immune system.

To test what happens to bacteria in orbit, a team led by James Wilson and Cheryl Nickerson, of Arizona State University, flew samples of Salmonella typhimurium, a major cause of food poisoning, on an Atlantis mission to the International Space Station, which launched on September 9 last year.

During the flight, one of the astronauts, Heidemarie Stefanyshyn-Piper, activated the experiment by pressing a plunger that opened up a chamber and allowed the bacteria access to growth media inside.

After 24 hours she pushed the plunger again, which either “fixed” the bacteria with chemicals that preserved the gene expression message, or else introduced fresh media to keep the bacteria growing to perform the virulence studies. In this way the “fixed” cultures were preserved for analysis back on Earth.

At the same time, a similar control experiment was conducted in a laboratory at Arizona State University.

The tubes of bacteria on board Atlantiswere sealed in a triple containment facility to ensure that they posed no risk to the astronauts.

Analysis at the end of the flight showed that the salmonella flown in space had changes in the expression of 167 genes. When used to infect animals, the space-grown bacteria proved to be three times more likely to cause disease than those grown on Earth.

The research, which is published in the journal Proceedings of the National Academy of Sciences, identified particular changes in gene expression that were linked to an important regulatory protein called Hfq.

This, the scientists suggested, could explain why microgravity led to increased virulence.

This insight could also lead to new approaches to developing antibiotics and vaccines for fighting salmonella infections.

Awareness of the potential extra risk to astronauts is particularly important because space agencies around the world are placing new emphasis on manned spaceflight. Nasa plans to return astronauts to the Moon by 2020, and then to build a permanent base there.

Two weeks ago, a government advisory panel recommended that Britain should end its long-standing refusal to fund manned spaceflight and start recruiting a four-person corps of British astronauts.

Professor Nickerson said: “Space flight alters cellular and physiological responses in astronauts including the immune response. However, relatively little was known about microbial changes to infectious disease risk in response to space flight.”

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